Coalescence of n-hexadecane oil drops (70-100 mu m in diameter) suspended inside an aqueous medium of varying pH and ionic strength is reported. The drops were produced in situ by micropipettes inside aqueous-phase-filled fine capillaries (100-160 mu m, i.d.). At constant pH values of 5.7 and 10.9 the ionic strength of the aqueous medium was changed with KCl so that the Debye length (kappa(-1)) varied between 1.6 to 362 nm and 0.36 to 14 nm, respectively. Visual observations showed coalescence when kappa(-1) was less than similar to 5.0 nm. The DLVO theory together with previously reported measurements of critical thickness of rupture of aqueous thin films also predicts coalescence under these conditions. The effect of change in pH at constant ionic strength (kappa(-1) = 5.0 nm) was also studied. While theory predicts coalescence at all pH values with constant kappa(-1), coalescence time increases with pH. At similar conditions of high ionic strength, the coalescence times at very low pH are lower than at intermediate pH by two orders of magnitude, suggesting that pH is more important than ionic strength in controlling the coalescence of the oil drops.